How does cross-hatch recommendations change for single vs double sided assemblies (parts on one or both sides of the board)?  Does it change with hand soldering?  [Given that heat is applied unevenly to the board with hand soldering.]  I ask as I had a design that warped noticeably after hand soldering--I'd call it reasonably dense, SMT with parts on only 1 side, 0.062" with 8 layers and 3 continous gnd planes (Mid1,Mid4,Mid6 as GND planes, out of Top/Mid1/Mid2/Mid3/Mid4/Mid5/Mid6/Bottom).



Shawn Upton, KB1CKT

Test Engineer

Allegro MicroSystems, Inc

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-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of Kenneth Wood

Sent: Thursday, July 30, 2009 7:45 AM

To: [log in to unmask]

Subject: Re: [TN] Hashing-Out Hatching



I would not use cross hatching in modern high-speed or RF designs, cross hatching is ok for designs that have no high-speed though.

We are to the point of taking the tightness of the prepreg weave into consideration

for impedance issues so a cross hatched reference plane is defiantly out of the question today.

To your question, the only issue I have ever seen, and continue to see, is warpage of unbalanced layers.

Use outer layer thieving pads and balance inner layers to avoid this and you will be ok.

Even the outer layer thieving is optional but I do it as often as I can. (some customers hate seeing it)

It's not very scientific, if you have a 6 layer PCB and layer 2 is a solid plane then make sure that layer 5 (at least) closely resembles.

Ken 

_____________________________________

Kenneth J. Wood

Saturn PCB Design, Inc.             

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www.saturnpcb.com